Method of adjusting a resistor by anodizing



July 14, 1970 F. w. CARROLL 3,520,783

METHOD OF ADJUSTING A RESISTOR BY ANODIZING Filed Feb. 10. 1967 2 Sheets-Sheet 2 Fig. 2A 0 k 250,

Fi .2c i E E Fig-'20 o-+ ::I 235ms L I |I MEASURE TIME I VOLTAGE,

INVENTOR FRED W. CARROLL BY WMm PATENT AGENTS United States Patent 3,520,783 METHOD OF ADJUSTING A RESISTOR BY ANODIZING Fred W. Carroll, Ottawa, Ontario, Canada, assignor to Northern Electric Company Limited, Montreal, Quebec, Canada Filed Feb. 10, 1967, Ser. No. 615,180 Int. Cl. C231) 9/00 US. Cl. 204--23 1 Claim ABSTRACT OF THE DISCLOSURE A circuit which is used to control the final adjustment of thin film resistors through alternate anodizing and measuring of the resistance, that provides a stabilizing period before the measuring interval, in order to allow any parasitic capacitance to charge, so as not to affect the accuracy of the measurement. Such a capacitance is formed across the resistor when immersed in the electrolyte with the anodized layer forming the dielectric between the conductive resistor and the electrolyte.

This invention relates to a method and apparatus for controlling the trimming of metal resistors by anodization, and is praticularly applicable for adjusting the value of thin film resistors during manufacture.

Thin film resistors may be produced by selective etching of a thin layer of metallic material which has been deposited upon an insulated substrate. As a practical matter, the accuracy of such a process yields resistors with a tolerance of approximately 10 percent. To improve upon this, and provide resistors having a tolerance of better than 1 percent, the metal film resistors have been subjected to an anodization process.

This involves initially constructing the resistor to have a value lower than the desired value. The resistor is then anodized in an electrolytic cell thus forming an insulating oxide on the surface thereof. This reduces the crosssectional area of the resistor which, in turn, results in an increase in its resistance. The anodization process is continued until the resistance has increased to a desired value. One method of trimming such resistors is described in the US. Pat. No. 3,148,129, issued Sept. 8, 1964 and invented by H. Basseches et al. The process described in this patent involves continuously monitoring the resistance of the resistor while the anodization is taking place. This process is complicated by the fact that the flow of current resulting from the anodization must be taken into account when determining the actual resistance value otherwise inaccurate values will result.

In order to overcome this problem, a further improvement has been made in which the resistor is alternately anodized and then measured immediately thereafter until the desired resistance value is obtained, whereupon the anodization process is automatically terminated. During the measuring step, current flow fom the resistor to the electrolyte and back into the resistor is prevented by the formation of the oxide coating over the surface. This coating may be obtained by a preliminary anodization carried out before the final adjusting process. This process has performed satisfactorily for producing resistors with a tolerance of 1 percent or better up to a value of about 10 kilohms. However, when it was attempted to produce resistors having a value higher than this, it was found very difficult to maintain the tolerance within the desired 1 percent. Thus, even though the source of anodization current had been disconnected from the electrolytic cell prior to measuring the value of the resistor, the measurement and could not be accurately made.

It has been discovered that this resulted from the 3,520,783 Patented July 14, 1970 formation of a parasitic capacitance wherein the conductive portion of the resistor and the electrolyte form the two conductors with the anodized layer acting as the dielectric. This provides an effective capacity shunting the terminal ends of the resistor. The parasitic capacitance does not appreciably affect low value resistors since the capacitive reactance is not low enough to alter the measured value of the resistor. However, when trimming high resistances, applicant has found that the charging time of this parasitic capacitance must be taken into account or erroneous measurements will be obtained. Thus, the shunting reactance of the capacitor results in an impedance measurement which is lower than the actual resistance value thereby prolonging the anodizing process. This ultimately results in the thin film resistance having a value higher than that desired.

Applicant has found that by providing a waiting or stabilizing period between the application of the measuring voltage and the monitoring of that voltage (so that the parasitic capacitance may charge), that this process may be extended to high value resistors without obtaining erroneous measurements. Such a process is then only limited by the leakage current which exists through the anodized layer and the electrolyte resulting from the potential impressed across the resistor during the measuring step.

In accordance with the present invention there is provided a method of adjusting the resistance of a metal resistor by anodizing in an electrolyte which comprises the steps of applying an anodizing current between the resistor and a spaced electrode immersed in the electrolyte so as to anodize the resistor and progressively increase its resistance. Thence, removing the anodizing current and applying a measuring voltage across the resistor. Monitoring the measuring voltage a predetermined period of time after its application to determine the resistance of the resistor. The predetermined period of time being sufficient to allow any parasitic capacity between the resistor and the electrolyte to charge to the measuring voltage. Thence, repeating the previous steps if the measured resistance is below a desired value, or terminating the anodizing when the measured resistance reaches the desired value.

An example embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 illustrates a schematic diagram partly in block form of an anodization control circuit is accordance with the present invention; and

FIGS. 2A, 2B, 2C, 2D, and 2E illustrate voltage waveforms at various reference points in the circuit illustrated in FIG. 1.

Referring to FIG. 1, there is shown a metal resistor in the form of a thin film tantalum resistor generally 10, having a conductive strip 11 deposited on an insulated substrate 12. The resistor 10 is shown immersed in an electrolyte 13 of an electrolytic cell, generally 14, and functions as an anode electrode during the anodizing process. Also immersed in the electrolyte 13 is a cathode electrode 15 which is spaced from the thin film resistor 10. An anodizing power supply 16 provides a constant source of anodizing current during the anodizing process as hereinafter explained. The anodization control circuit comprises a control means in the form of a freerunning multivibrator 20, an output of which is used to drive a relay coil 21. In addition to driving the relay coil 21, the multivibrator 20 drives two monstable multivibrators 22 and 23 which in turn control two additi nal monostable multivibrators 24 and 25 respectively. The output of the monostable multivibrator 24 drives a relay coil 26, while the output of the monostable multivibrator 25 is connected to one input of an AND gate 27. The

3 anodization control circuit also comprises a measuring means which includes a resistance bridge, generally 30, in which the thin film resistor forms one arm. The other three arms are formed by two fixed resistors 31 and 32 and a variable resistor 33. The measuring current for the bridge 30 is supplied by a D-C bridge power supply 34 which is connected across opposing ends of the bridge 30 through normally closed control contacts 35-1 of a relay coil 35. The other two ends of the bridge 30 are connected to a D-C amplifier 36, the output of which is connected to a second input of the AND gate 27. Connected across the input of the DC amplifier 36 are normally closed shorting contacts 26-1 which are controlled by the relay coil 26.

The output of the AND gate 27 is connected to one input of an OR gate 40, the output of which is used to drive the relay coil 35. In addition to controlling the normally closed contacts 35-1, the relay coil 35 also controls normally open lock-up contacts 35-2 and normally closed contacts 35-3. The normally open contacts 35-2 are connected in series with a reset switch 41 between a second input to the OR gate 40 and a source of voltage 42. To indicate when the anodization process is completed, an indicating lamp 43 is powered from the source of voltage 42 through the normally opened contacts 35-2.

The measuring arm of the bridge 30 is coupled across the thin film resistor 10 through connecting means which in the present embodiment comprise normally closed contacts of break before make transfer contacts 21-1 and 21-2 that are actuated by the relay coil 21. The negative terminal of the anodizing power supply 16 is connected directly to the cathode electrode 15. The positive terminal of the supply 16 is connected through the normally closed contacts 35-3 and normally open contacts of transfer contacts 21-1 and 21-2 to the thin film resistor 10.

Referring generally to FIGS. 1 and 2, the voltage output of the free-running multivibrator 20, reference point A of FIG. 1, is illustrated graphically in FIG. 2A. As shown, the output is a square wave having a total period of 500 milliseconds. The voltage at reference point A is used to drive the monostable multivibrator 22 which produces at its output, reference point B of FIG. 1, a pulse of 230 milliseconds as shown in FIG. 2B. This in turn is used to drive the monostable multivibrator 24 which produces at its output, reference point C of FIG. 1, a driving pulse of 12 milliseconds as shown in FIG. 2C.

The voltage output at reference point A is also used to drive the monostable multivibrator 23 which produces at its output, reference point D of FIG. 1, a pulse of 235 milliseconds as shown in FIG. 2D. This in turn drives the monostable multivibrator 25 which produces at its output, reference point E of FIG. 1, a gating pulse of 3 milliseconds as shown in FIG. 2B.

During operation, the voltage output at reference point A, from the multivibrator 20, actuates the relay coil 21. This in turn transfers the thin film resistor 10 from the bridge to the positive terminal of the anodizing power supply 16 by actuation of the transfer contacts 21-1 and 21-2. With the anodizing supply 16 connected across the electrolytic cell 14, anodization of the tantalum thin film resistor 10 takes place for a time interval of 250 milliseconds. This converts part of the conductive portion of the strip 11, consisting of tantalum nitride, to tantalum pentoxide which is an insulator. This slightly reduces the cross-sectional area of the resistor 10 thereby increasing its overall resistance.

At the end of the 250 millisecond interval, the relay coil 21 is de-energized which in turn disconnects the anodizing power supply 16 and reconnects the thin film resistor 10 to the measuring arm of the bridge 30. The bridge power supply 34 which, at this point, is connected across the bridge 30 through the normally closed contacts 35-1, applies a voltage across the th n film resistor 10. Because, at this point, the contacts 26-1 are closed, any unbalance in the bridge 30 will not be immediately coupled to the amplifier 36. Hence, current flow, resulting from the charging of any parasitic capacitance, will not affect a true reading of the resistor 10. After a stabilizing period of 230 milliseconds, the pulse signal voltage at reference point C energizes the relay coil 26 thereby removing the shorting contacts 26-1 and allowing any unbalance voltage from the bridge 30 to be applied to the D-C amplifier 36 for a period of 12 milliseconds. This voltage is amplified and the output coupled to one input of the AND gate 27. Five milliseconds after the removal of the shorting contacts 26-1, the pulse at reference point E is applied to the second input of the AND gate 27 for a period of 3 milliseconds. If the resistance value of the thin film resistor 10 is below the desired value, the output of the D-C amplifier 36 will be opposed to that at reference point B and no output signal will be coupled from the AND gate 27. As a result, when the control signal at reference point A reactuates the relay coil 21, the bridge 30 will again be disconnected from the resistor 10 and anodizing power supply 16 will be reconnected across the cell 14.

With the power supply 16 reconnected, the anodization process continues with further conversion of the outer layer of the conductive portion of the thin film resistor 10 to an insulator. This is turn further decreases the cross-sectional area of the resistor 10 thereby resulting in an additional increase in its overall resistance.

This alternate anodizing and measuring process continues until the resistance of the thin film resistor 10 has risen to a value where the bridge 30 passes through its null or balance point. As a result, the polarity of the output voltage connected to the D-C amplifier 36 is reversed during the time interval that the contacts 26-1 are open. Hence an output signal from the amplifier 36 is coupled to the AND gate 27 that is in phase with that of reference point E thereby opening the gate 27. This in turn opens the OR gate 40 which provides a drive signal for the relay coil 35. Actuation of the relay 35 closes the lockup contacts 35-2 which in conjunction with the reset switch 41 applies a holding voltage from the source 42 to the OR gate. Actuation of the relay 35 prevents further anodization and measurement of the thin film resistor 10 by disconnecting the power supplies 16 and 34 through opening of the contacts 35-3 and 35-1 respectively. The adjusting process is now terminated as indicated by the control lamp 43 which receives its voltage from the source 42 through the contacts 35-2.

A new thin film resistor may now replace the resistor 10 and the adjusting process restarted by depressing the reset switch 41. The process then continues in a like manner as described above until the value of the new resistor reaches the desired level whereupon the process will again be terminated.

The gain of the D-C amplifier 36 and the sensitivity of the AND gate 27 are designed so that a resistance value for the thin film resistor 10 of about 0.05 percent above the desired value provides sufiicient voltage to open the AND gate 27 during the 3 millisecond time interval that the pulse appears at reference point E. In addition, the anodizing time as determined by the free-running multivibrator 20 and the current from the anodizing power supply 16 are adjusted so that the total change in resistance value during each anodizing cycle is held to 0.2 percent.

It has been found that a parasitic capacitance in the order of 0.1 microfarads can be formed across the thin film resistor 10 with the conductive portion of the resistor and the ionized electrolyte 13 acting as the two conductors and the anodized layer on the thin film resistor 10 acting as the dielectric. Thus, when adjusting a resistor for a final value of 200 kilohms to 1 percent accuracy, applicant found that a stabilizing period of 230 milliseconds, as determined by the output from the monox stable multivibrator 22, was required to allow adequate time for the parasitic capacitance, resulting from the application of voltage from the D-C bridge power supply 34, .to charge before the contacts 26-1 are opened and the resultant voltage from the bridge 30 is connected to the D-C amplifier 36. Appreciable variations in the value of the parasitic capacitance may be found depending upon the geometry and configuration of the resistor 10 and the thickness of the anodized layer thereon. Hence, it may be necessary to use longer stabilizing periods depending upon the final value of the resistor and the accuracy required.

This system permits the trimming of high value thin film resistors in the megohm range within 1 percent tolerances or better. The upper limiting factor for the system will be limited by the leakage current through the oxide into the electrolyte 13 which results in an impedance shunting the thin film resistor 10. The lower limit for trimming resistors by this method is primarily dictated by the contact resistance of the anodization control circuit. Additionally, to prevent appreciable anodizing during the measuring step due to the bridge power supply 34, the voltage therefrom is chosen to be Well below that supplied by the anodizing power supply 16 during the anodizing interval.

While the anodizing process has been described for trimming thin film resistors made of tantalum nitride, it can be readily extended to resistors made of any other suitable material which will anodize and for which it is desired to adjust the resistance value thereof.

What is claimed is:

1. A method of adjusting the resistance of a metal resistor by anodizing in an electrolyte comprising the steps of:

(a) applying an anodizing current between the resistor and a spaced electrode immersed in the electrolyte for a first period of time to anodize the resistor and thereby progressively increase the resistance thereof, the anodization resulting in a capacitance being formed between the resistor and the electrolyte;

(b) removing the anodizing current and applying a direct current measuring voltage across the resistor for a second period of time;

(c) allowing the resultant direct current to flow until the charge across the capacitance reaches equilibrium;

(d) after said equilibrium is reached, feeding the measuring voltage to a monitoring circuit to determine the resistance of the resistor;

(e) repeating steps (a), (b), (c) and (d) if the measured resistance is below a desired value; and

(f) terminating the anodizing when the measured resistance reaches the desired value.

References Cited UNITED STATES PATENTS 3,282,821 11/1966 Cistola 204-228 3,341,444 9/1967 La Chapelle 204228 3,341,445 9/1967 Gerhard 20428 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-l4l 

